Zinc is widely used in many different fields (e.g. building industry, domestic appliances, mechanics), especially as a coating for its protection properties: zinc acts as a sacrificial anode. However its use is somehow limited by the inclination to the oxidation and corrosion when exposed to air. For the same reason zinc never found a wide diffusion in electronic applications.
In the electronic field, more specifically in the manufacture of electronic modules, patent application EP-A-97300569.7 discloses a method of pretreating a copper surface by applying a thin film of zinc with the purpose of improving the solder property of tin based solder alloys. Zn is believed to be usually harmful for a soldering process if present in a Sn alloy. The result is that the wettability of the alloy is decreased, because Zn is highly reactive with Oxygen and during reflow produces Zn oxide which compromises the connection between the pins of the electronic component and the solder alloy and prevents solder coalescence. However if a thin Zn layer is deposited onto the Cu substrate before depositing the solder alloy, during the reflow it does not come in contact with oxygen and it does not oxidize. A very thin Zn layer is able to significantly improve the mechanical and electrical properties of the solder layer which is hundreds of time thicker. This effect seems to be mainly due to the ternary alloy CuZnSn which is formed near the surface of the Cu substrate. Studying the thermodynamic behaviour, it is possible to see that the Sn and Zn attractive interaction for Cu atoms is in some way increased by their mutual repulsion, influencing the valence band, through possible orbital hybridization, and increasing the ternary alloy stability. The increase of the number of states at the Fermi energy level could account for a possible increase of the interface electrical properties.
Another important effect of the Zn pretreatment of the Cu substrate is related to the aging behaviour of the coating. Artificial aging experiments have shown a good behaviour of the ternary alloy CuZnSn obtained with the above method, due to the high deposit uniformity if compared with other Pb free solder methods.
According to the method described in EP-A-97300569.7 the zinc film is deposited on the Cu surface by electro deposition. The electro-deposition process has the drawback of requiring electrical connection to the area where the treatment is needed and this can cause some problems in many case. Patent application GB-9800624.0 discloses a method for an electroless deposition of Zn--Sn layer on a metallic surface. This film improves the solderabilty properties of a Sn alloy and it is particularly useful in the manufacturing of electronic packages, when metallic components with Cu contacts must be soldered to a substrate having Cu circuit traces by means of a Sn based alloy (e.g. Sn--Bi alloy).
However, the zinc coating described above presents the drawback of being very vulnerable to the atmospheric agents. For this reason the solder alloy must be immediately deposited on the zinc film in order to avoid the corrosion and the oxidation caused by its exposure to the air.
It is an object of the present invention to provide a technique which alleviates the above drawbacks of the prior art.